Friday, 6 December 2013

cvgtMM - A ModuleModule version of the CVGT1

This project has kept me busy for the past few months. It was suggested some while ago that I do a ModuleModule version of CVGT1 to go in a Buchla (see my earlier post). My initial thoughts were that it was probably not the best solution as front panel space in a ModuleModule is quite restricted and Buchla real-estate is not cheap. However what I didn't consider was that Buchla can also talk to Serge, BugBrand, Fenix etc and Serge users seemed to want this solution. Consequently I set about designing (or redesigning) CVGT1 to fit in a ModuleModule form factor. Chris Muir from Eardrill produced a great interface document which defines just about all I needed to know for designing a ModuleModule (I'll refer to ModuleModule as MM from now on). 

Because of the smaller front panel space I had to lose some features i.e. scaling and offset on the Buchla to other synth CV channel but as it is in a Buchla and will interface to an external 1V/octave synth I've fixed the scaling at 0.8333 (1/1.2) and zero offset. The PCB design allows either 3.5mm jacks or 4mm bananas to be fitted; so one PCB can be configured to for Euro or Serge etc.

Clearly this design has PCBs parallel to the front panel unlike CVGT1 which has them perpendicular hence the change in jack style.  The PCBs had a completely new layout with all front panel related parts on the first board (Front Panel Interface Board) and most of the circuitry on the second (Main Board). Here are some design images of the variants (there are a few very minor tweeks to the graphic since this was drawn and you'll see a few variations in the prototype pics below).

I built a prototype and that's all working nicely. Aside from the new PCB layouts I had to make some changes to the pulse/trigger/gate circuits to run from ±15V rather than ±12V but there were no major changes. Heres a scope image of the Buchla compatible pulse I achieved:-

Here's the prototype with an ink jet printed plastic overlay on the front panel.

All components are conventional through-hole types - I may go over to surface mount if I get more orders than I can personally cope with but think I will still retain through-hole if anyone fancies a kit. You will also notice that the scaling trimmers are front-panel accessible (with a small screw driver) however these are for very fine trimming ±0.5%.

The first pre-production modules will be available in just over a week when the screen-printed panels are in stock. I have a number of interested 'customers' that I will contact soon and will start a pre-order list; please feel free to contact me to get on the list: It will be a 'no-commitment no money up front' arrangement as before with CVGT1 and I will poll all interested parties in turn when I have a module ready to ship to see if you want one. I have not finalised a price yet but guess it will be in the same ballpark as Eardrill's own range of single panel medium complexity MMs.

It will be interesting to see how this takes off, CVGT1 started slow and now there are 200 out there with bags of interest still.  I will do an update in the next week or so. Thanks for reading.